Homebrew Digital Sampling Audio spectrum analyser.

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http://www.serasidis.gr/circuits/AVR_oscilloscope/avr_oscilloscope.htm

DSC06118_.jpg
would be neat to have a standalone unit... not needing a scope.
 

GK

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Joined 2006
OK, an update.

I have finished revising the design of the digital sampling board and am 70% through drawing the schematic in Protel.

I have also decided to take a different approach with the design, after realising that this section of the Analyser would be a very useful piece of test equipment on its own, essentially being a self-contained low frequency digital storage adaptor of an analogue oscilloscope.

Anyone who has used an analogue oscilloscope to look at low frequency waveforms would know what a waste of time it is, especially when the horizontal sweep rate is so slow that the persistence of the display makes the wriggling dot look like a little comet flying across the screen.

I have named this self-contained DSO adaptor the DSOMK1, and have added it to the contents page of my website (currently under development) under Test Equipment:

http://users.picknowl.com.au/~glenk/HOME.HTM

The DSOMK1 will allow nearly any analogue oscilloscope to display low frequency signals, recorded at horizontal sweep rates from 0.1 seconds per division to as low as 50 seconds per division.

The nearly finished schematic of the DSOMK1 is attached below. All I still have to add is the PIC16F876 microcontroller and control interface on the left of the page and a buffer opamp for the ADC input.

The operation of the unit is very simple. There are two 1k 8 bit static memory chips. At any one time, one will be put in read mode for generating the display on the oscilloscope CRT while the other is in write mode for recording the input signal at the selected sweep rate of 0.1s to 50s per division.

While recording, the display memory is continuously clocked completely through its 1024 bytes at a rate of 25 times per second, in synchronisation with the 10 bit ring counter and R/2R DAC at the bottom of the schematic that generates the 25Hz saw tooth horizontal sweep waveform, fed into the X amplifier input of the oscilloscope.
The output of the memory chip being read is decoded by an 8 bit R/2R ladder DAC and sent to the oscilloscope vertical input.

This provides a flicker fee, static display of the last recorded long-duration sweep on the oscilloscope display until the microcontroller has completed recording with the other memory chip. Then the other memory chip with the fresh recording is switched into display mode by the microcontroller, while the next sweep is then recorded into the memory previously used for display. This process repeats continuously.

The horizontal sawtooth sweep output for the oscilloscope X input is +/-2.5V peak while the Y (vertical) output for the oscilloscope Y input is +/- 2V peak, with -10V 1mS pulses during the horizontal retrace interval to ensure that the trace is deflected off screen to prevent visible retrace during this period.

These voltages ensure that the display fro the DSOMK1 will fill the typical 10cm (Horizontal) by 8cm (vertical) display of the analogue oscilloscope with the variable positions controls centred and both the horizontal and vertical deflection amplifiers set to 0.5V per cm/division.

I hope I've explained that clearly :xeye:

Anyway, the design uses cheap garden variety parts, the semiconductor complement being:

One PIC16F876 (The master controller)
One 20MHz 8 pin oscillator module (heartbeat for the PIC)
Two CY62256 SRAM (the memory, only addressed for 1kbyte)
Three 74HC4040 (for RST synchronised address counters)
Two 74HC573 and two 74HC574 (for MUX'ing the ADC and the DAC to the alternate memory chips)
One ADC0804 (8 bit ADC)
One LM336-2V5 (Voltage reference for ADC and voltage level shifting of X/Y output signals)
One TL081 (input buffer for ADC)
One TL074 (X/Y output)

Cheers,
Glen
 

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GK

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Joined 2006
Dimitar13 said:
Mr Kleinschmidt,

Could you , somehow, add pictures with higher resolution ? The pictures you have attached do not have high enough resolution to see the details i.e. symbols, values, letters.

thanks in advance
Dimitar


Hi Dimitar

Those pics are just for a bit of a progress report. Detailed schematics and all details (uC code, etc) will go on my website once the design is built and tested and debugged.

This may take a while though, but I am eager to get the complete design up an running to conduct some detailed measurements (paired with a THD analyser) of a series of power amps I'm building, so it does atleast have a reasonably high priority ;)

Cheers,
Glen
 
G.Kleinschmidt said:



Hi Dimitar

Those pics are just for a bit of a progress report.

AKA teaser :D

Nice project mr. Little Smith!

I'm also sure its going to be pretty cost effective (although $200 seems a little optimistic).

BTW, are you aware how much is a TDS 60MHz digital scope, dual channel, B&W display, with 1Gs/sec sampling rate, built by Tektronix? Brand new, $1700 plus taxes and shipping. And you can pick one from EBay, still under warranty, for about $700 (or even less if you are lucky).
 

GK

Disabled Account
Joined 2006
Thought I'd bump this thread again with a progress update. The design is complete on paper, I just need the time to finish building it.
However I have also been busy finalising and building a prototype of a completely different type of audio spectrum analyser. It is a revamp of a design of mine from nearly 10 years ago that displayed the measured audio spectrum on a television display.

The new design has a VGA video output (31.250kHz Hor, 60Hz Vert.) which is compatible with any computer monitor.
The display generated on the monitor is as shown in the diagram attached to this post. The complete audio spectrum is divided into twenty (half octave – 1.414) frequency bands, each represented on the screen in real time by a red, green, blue or yellow level bar.
A 10 by 20 square graticule is drawn over the top and the vertical scale is logarithmic at 3dB per division, giving the display a dynamic range of 30dB.
 

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GK

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Joined 2006
Attached below is a photo of how far I got this weekend. Shown is the digital section of the unit plugged into a SVGA monitor. The photo didn’t come out very well due to the camera flash, but the early stages of the generated display can be seen.

The digital board has an analogue multiplexer with 20 voltage inputs (one for each frequency band). Each input will be fed from a simple half wave peak detector preceded by a two stage MF bandpass filter.
I haven’t built the analogue/filter board yet. In the photo below I am just feeding one of the analogue inputs from the wiper of a potentiometer wired across the supply rails.
The variable voltage from this potentiometer is producing the yellow level bar that can be seen on the screen.

The white graticule can also be seen. The graticule currently fills the entire screen without a border, as I haven’t finished soldering in the vertical and horizontal video blanking logic yet.
When finished the graticule displayed on screen will be comprised of just the 20 by 10 square grid pattern as described in the previous post.
 

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GK

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The design uses garden-variety and cheap parts throughout - 74HC series CMOS logic chips, a LM360 comparator, TL07X series opamps, a 4MHz parallel resonant crystal and a LM336-2V5 reference diode.

The logarithmic response of the level bars is produced by comparing each analogue input as each horizontal line is drawn against the exponential voltage on a capacitor which is charged during each frame and discharged during the vertical blanking/retrace interval.

The 20 channel two stage multiple feedback bandpass filter / peak detector board will use 15 TL074 opamps. The simulated response of the 20 channels is shown in the diagram attached below. Passband ripple is 4dB.

When I've completed the protoype full curcuit details of the design will go on my website.

I am interested in laying out boards for the design and building myself a version with professionally made PCB's, so I'm wondering what kind of interest there may be out there for such an instrument.

I would need about 20 people to put their name down for a set of boards (one analogue, one digital - sold by me at cost) to make that viable.

Cheers,
Glen
 

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GKlein.

I am interested.
I know you are a very clever designer.

1. I want to know how I attach this to my PC ( Windows XP )
I have USB 2 and Firewire 800 ports.

2. What is the bandwidth of input signals

3. you write the software your self. Or some friend?


Example of use I could think of:
Attach several testpoints to my amplifier under test
and get a fair view of what it s going on in different stages.
( is enough if I can get the audio band frequency range = 20 - 20.000 )

More info to me please.
I know this project is far more interesting than diyaudio people have yet discovered.


Lineup
 

GK

Disabled Account
Joined 2006
lineup said:
GKlein.

I am interested.
I know you are a very clever designer.

1. I want to know how I attach this to my PC ( Windows XP )
I have USB 2 and Firewire 800 ports.

2. What is the bandwidth of input signals

3. you write the software your self. Or some friend?


Example of use I could think of:
Attach several testpoints to my amplifier under test
and get a fair view of what it s going on in different stages.
( is enough if I can get the audio band frequency range = 20 - 20.000 )

More info to me please.
I know this project is far more interesting than diyaudio people have yet discovered.


Lineup



Lineup, this is a stand alone unit that just plugs into any PC monitor for the display. No PC or software required. :)
 
you see, GKlein
I have 2 older PC monitors not in use no more.
( I use a flat LCD monitor for my PC )

One of them monitors is not very old. ( 17 inch )
I guess I can use this PC monitor standard cable for connection.
It will most probably fit your spectrum anaylyser output.
 
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